[Bldg-sim] Modelling simplification for structural concrete columns

[Wealend, Edwin]

Patrick,

Those are some mighty columns!

I think it depends on what you're trying to achieve with the model. If you're looking at radiant temperatures, local thermal comfort or similar, then I would agree that taking an area weighted approach is going to neglect important local differences due to thermal inertia and other factors. However, if you're looking at the energy use of the entire building, and it's conditioned 24 hours, or in a climate with negligible diurnal swing, or with limited thermal mass generally, I would still say that a simplified approach would give you results that are accurate enough. I concur with Joe on the point about thermal bridging. Particularly if the remainder if the façade is insulated. Most construction inputs in energy modelling software don't accurately account for thermal bridging, so if you think they're going to be significant, then use a separate 2D calculation software and adjust your u-values appropriately.

That said, as they're relatively large and it's easy enough to divide up the walls into separate constructions, as it is with software like IES, then by all means model them separately and apply a different construction.

Much of the importance of the above is also dependant on the climate your build is in. E.g. Un-insulated concrete columns in a northern European climate are a terrible idea and will come with a big energy penalty. In warmer climates, their contribution to the overall energy of the building will be much smaller and your efforts on the accuracy of their modelling may be better spent looking at something like the fresh air or building leakage.

Regards,
Ed

Edwin Wealend
Senior Engineer

CUNDALL Hong Kong
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From: Patrick Bivona [mailto:patrick.bivona at gmail.com]
Sent: Friday, March 15, 2013 14:53
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Modelling simplification for structural concrete columns

Joe, Edwin,

I take your point that I'm probably too literal in my attempt at modelling the geometry of this wall.

Thanks for the collegial suggestion about THERM. It seems really good and I'm pretty sure it's going to be useful to me further down the line.

The building I'm modelling is a tower with a square floor section of 1400m2. There are only a few uninsulated columns but they are relatively big (1.2m wide, 2m deep) and spaced by 8m or 18m. It doesn't seem appropriate to me to give an overall U-value to the assembly, spreading the effect of the columns over such "long distances". I'd rather model the columns as surfaces separate from the  surrounding walls, especially as they are glazed curtain walls.

Patrick

---------- Forwarded message ----------
From: Joe Huang <yjhuang at whiteboxtechnologies.com><mailto:yjhuang at whiteboxtechnologies.com>>
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Cc:
Date: Thu, 14 Mar 2013 01:00:07 -0700
Subject: Re: [Bldg-sim] Modelling simplification for structural concrete columns
Sorry, caught a couple of typos, meant to write in the second paragraph, "...even if it doesn't jive.."  and "they can be modeled..."

Joe

On 3/14/2013 12:35 AM, Joe Huang wrote:
Patrick,

I frankly think that you're looking at the modeling of building geometry in simulation programs too literally.  Please keep in mind that in the end all
the simulation programs are just doing hundreds of one-dimensional heat flow calculations through thousands of time steps.  Wall thickness is needed
only to compute the heat flow characteristics (U-value, response factor, transfer function, etc.) but is not modeled explicitly in the simulation, i.e.,
walls have no thickness in the simulations.  Similarly, the space volume is used only to determine the amount of air participating in the space heat balance, so therefore you should always input the actual enclosed volume, even if it does jive with the dimensions of the enclosing surfaces.  If you're concerned about the shading effect of the columns, they can be model them as narrow vertical building shades with widths equal to the protruding part of the columns, but if the walls are opaque, the effect must be extremely small.

However, I think you're ignoring  what I think is the most significant heat transfer aspect of these columns, which is the two-dimensional conduction through them, because the columns act like stubby fins on the wall, particularly if they're solid concrete with no insulation.  That's where I would concentrate my energies if I wanted to get the modeling right.

To do that, you would really have to model the wall-column-wall assembly with a 2-D conduction program like THERM (available from LBNL), although that was written for modeling window frames and can only output a steady-state U-value.  I still have a 2-D conduction program written in Fortran called WALFERFN (sounds German, doesn't it ? but the acronym stands for WAll Finite Element Response Factor New :-) ) that I use to calculate response factors for composite walls, but that can only handle planar surfaces.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com<mailto:yjhuang at whiteboxtechnologies.com>
www.whiteboxtechnologies.com<http://www.whiteboxtechnologies.com>
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
For further information about Cundall please visit http://www.cundall.com/Locations/Asia/Asia.aspx
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